Standalone wave meters are well known. Standalone wave meters are used to determine the wavelength of light incident upon a detector thereof. Standalone wave meters have different applications, including the testing and calibration of monochromatic light sources. Examples of such monochromatic light sources include lasers which are used in dense wavelength-division multiplexing (DVDM) fiber optic communications systems. Standalone wave meters may also be used to determine the wavelength of light which is output from lasers in a wide variety of other applications.
Although such standalone wave meters are useful for many different purposes, they are not typically integrated permanently into DWDM fiber optic communications systems for maintaining the desired channel center frequencies of the lasers of such system because of their size and cost. As those skilled in the art will appreciate, such standalone wave meters are typically too large to be permanently integrated into such DWDM fiber optic communications systems. Such standalone wave meters are also comparatively expensive, and thus cannot be deployed on a wide scale within such systems.
DWDM is considered to be the key technology to upgrade existing fiber optic networks to higher transmission capacities. Those skilled in the art will appreciate that an increase in demand for data carrying capacity can be accommodated by increasing the number of wavelength channels on a single fiber via the use of DWDM. Such enhancements in the data carrying capacity of an optical fiber communications system is a cost-effective alternative to laying new fiber in the ground.
Presently, the total usable bandwidth of a DWDM fiber optic communications system is limited by the bandwidth of erbium-doped fiber amplifiers. Because of this total usable bandwidth limitation, data carrying capacity must be enhanced by more effectively utilizing this available bandwidth. The total usable bandwidth can be more efficiently utilized by decreasing the channel spacing of the wavelength-division multiplexing fiber optic communication system. However, precise control of each channel center frequency must be maintained in order to avoid channel cross talk, which would otherwise inhibit utilization of such decreased channel spacing.
The International Telecommunication Union (ITU) specification is presently set to a 100 GHz channel spacing (equivalent to 0.8 nm), but a move toward 12.5 GHz (0.1 nm) is expected in the near future. To avoid channel cross talk in such a DWDM system, the laser wavelength should be kept to one specified ITU standard frequency of the grid. A wavelength tunable laser, such as a distributed feedback (DFB) laser or a distributed Bragg reflector (DBR) laser, in cooperation with a wavelength monitor which can monitor the laser's wavelength and provide feedback to lock the laser wavelength onto one of the wavelengths of the ITU grid standard, is essential to the success of future DWDM systems.
DFB lasers and DBR lasers appear to be candidates for laser sources in DWDM fiber optic communications systems because of their single longitudinal mode operation, wavelength tunability, and low cost. Generally, their wavelengths can be tuned over several nanometers using temperature adjustment. A wavelength tuning range covering the whole C-band (1530 nm–1565 nm) can be achieved with, for instance, a laser array. Also, DFB lasers and DBR lasers with wavelengths in the S-band (1480 nm–1530 nm) and the L-band (1565 nm–1600 nm) have been studied extensively.
One common way of locking the laser wavelength onto the ITU grid involves using a Fabry-Perot etalon. The etalon has a periodical wavelength response as shown in FIG. 1. The period of the wavelength response (free spectrum range, FSR) is beneficially designed to be the DWDM channel spacing. For example, the etalon in FIG. 1 has a FSR of 50 GHz (0.4 mm). In such applications, the wavelength of maximum signal slope is beneficially designed to be at the ITU grid wavelength, so that this etalon can be used to lock the laser wavelength onto the ITU grid.
The capture range is defined as the range over which the laser wavelength can be accected and brought to lock onto one specified wavelength, for instance an ITU wavelength of the grid. In this application the capture range is defined by a continuous monotone portion of the response curve (which determines the etalon's output signal) which has sufficient slope. Generally, the locking range of a wavelength locker based on a single etalon is less than half of its FSR. Common methods of increasing the practical locking range include the use of a temperature controlled wavelength locker and the use of dual-etalon wavelength locker.
The drawbacks of such wavelength lockers which are based on a single etalon include channel ambiguity. Because of the inherent periodical wavelength response of the etalon, it is not possible to distinguish which period (that is, which wavelength channel of the grid) the laser wavelength is locked onto based only on the information provided by the wavelength locker itself. Other means are required to determine the channel number at which the laser wavelength is locked. For example, the channel number may be identified by the laser temperature in the case of temperature tuned DFB lasers and DBR lasers.
However, in some cases, especially for systems using fast switching, the channel number (coarse wavelength) of the laser may not be provided, so other methods for channel identification are required.
Another method of wavelength locking without channel ambiguity has been demonstrated and is based on the use of two narrow bandpass filters (BPF). FIG. 3 is a schematic diagram of such a wavelength locker based on two narrow bandpass filters (BPF). The laser beam is split with a beam splitter and then sent through two BPFs. As shown in FIG. 4, the wavelength response curves of the BPF is beneficially a Gaussian curve, the bandwidths of the two BPFs are beneficially designed to be the same, and only different peak transmission wavelength are used. The difference or the logarithms of the two filters' output is beneficially the feedback input to the laser wavelength control loop.
The advantage of this scheme is that as long as there is sufficient overlap of the two response curves, the laser wavelength can be locked to a wavelength between the peak wavelengths of two filters (FIG. 4) or any wavelength at which sufficient signal is provided in both channels (P1 and P2 in FIG. 3 and in FIG. 4) respectively. However, the slope efficiency, which is inversely proportional to the peak wavelength difference, is generally low, which will limit the wavelength locking precision.
Combining the two methods mentioned above, using the dual BPF as the channel identification means to bring the laser wavelength within the capture range of the etalon, and subsequently locking the laser's wavelength onto the specified ITU grid with the etalon, may provide a wavelength locker without channel ambiguity and can provide a wavelength locker with high slope efficiency. However, isolated optics components of the above-described methods are difficult to assemble and package.
Wavelength monitors based on integrated optical circuits (10C) have also been demonstrated. Integrated wavelength monitors include Mach-Zehnder interferometers and arrayed waveguide gratings (AWGs).
As shown in FIG. 5, an arrayed waveguide grating (AWG) generally consists of L input waveguides 501, an input slab waveguide 502, M intermediate waveguides 503, an output slab waveguide 504, and N output waveguides 505. Wherein L is 1 or more, M is 2 or more and N is 1 or more. The non-slab waveguides, 501, 503 and 505, are single mode waveguides. They are coupled to the slab waveguides 502 and 504 at the input and output sides thereof. The slab waveguides, 502 and 504, have symmetric curved input and output sides, such that for a central input the pathway length to any of the outputs are equal, as well as for a central output the pathways from all inputs are equal. All single mode waveguides coupling to a slab waveguide on one side are tangentially directed to the center of the opposing side.
Note that in the case of only one or two waveguides (FIG. 6) on a side there is no basis for a curved end of the slab waveguide and a simpler straight side may be implemented with no loss of functionality. Furthermore, based on the specific input used there will be a strongly monotone change in phase with lateral position at the output side of the slab waveguide except for the special case of a central input (FIG. 6) for which the phase is constant over the lateral position at the output side.
Light comes in from one of the input waveguides 501 and is transmitted across the input slab waveguide 502. In the input slab waveguide 502 the light will disperse with propagation and then couple into the intermediate waveguides 503. The intermediate waveguides 503 in turn are arranged such that for each pair of directly adjacent waveguides the effective optical path length difference is non-zero and is the same, such that the optical path length changes monotonically with lateral position (from one intermediate waveguide 503 to the next adjacent.) In general, if individual intermediate waveguides 503 are labeled consecutively with an index i, for one with neighboring waveguides on both sides its length and the length of the directly adjacent waveguides li, li−l, li+l, are related as follows: li−li−l=li+l−li.
The light travels from the intermediate waveguides 503 into the output slab waveguide 504. Upon propagating through the output slab waveguide 504, light is dispersed towards the output waveguides 505. Upon entering the output waveguides 505, light having emerged from the various intermediate waveguides 503 interferes. Since the output waveguides support only a single mode, only positively interfering components of the input will be transmitted through the output waveguides 505. Due to different geometric arrangements, different output waveguides will have different wavelength responses. The coherent addition of the contributions of the waveguides is equivalent to a properly used grating where the contributions from each of the individual steps coherently add, thereby justifying the often used term “grating waveguides” for the intermediate waveguides 503.
In considering directly adjacent intermediate waveguides 503, the effective optical path length difference accumulated upon traveling through the waveguides 501, 503 and 505 and the slab waveguides 502 and 504 has to be equal to an integer number of full wavelengths for intensity maxima to occur at the output of the waveguides 505. Such integer number is referred to the “order” of the grating.
Integrated optical circuits have the advantages of being easy to assemble and package. However, wavelength meters based on integrated optical circuits covering a relative large predetermined range (for instance the whole C-band), and providing wavelength precision of approximately 1 GHz have not yet been demonstrated. Therefore, it would be beneficial to provide wavelength meters based on integrated optical circuits which cover the whole C-band, and which provide wavelength precision of approximately 1 GHz.